1. Field of the Invention
This invention relates to a method for selectively diffusing impurities such as zinc into the substrate of a compound semiconductor such as gallium arsenide (GaAs) to provide well defined implantation areas.
2. Description of the Prior Art
Recently, semiconductors making use of compound semiconductors such as gallium arsenide (GaAs) have been put into practical use. In the manufacture of such devices, it becomes necessary to effect selective diffusion of impurities into the substrate.
In a typical example from the prior art, an SiO.sub.2 insulating film was deposited by chemical vapor deposition (CVD) on the GaAs substrate, and a diffusion window was formed in the film as by means of photoetching. By using the SiO.sub.2 insulating film as the selective diffusion mask, impurities such as zinc were diffused into the substrate for forming the required diffusion zones. However, the impurities tended to be transversely diffused over a substantial distance at the interface between the SiO.sub.2 film in the vicinity of the diffusion window and the GaAs substrate, creating an abnormal diffusion which made it difficult to control the size of the extent of the diffusion area. By "abnormal transverse diffusion" it is meant that the ratio of the transverse diffusion distance in the vicinity of the mask-substrate interface is large with respect to the vertical diffusion depth. If the transverse diffusion distance is denoted by the symbol x and the vertical diffusion depth by y, the ratio .alpha. is expressed as: ##EQU1## In the case of the prior art, the ratio .alpha. is larger than unity and in the case of an SiO.sub.2 film, the value of .alpha. is frequently equal to more than 4.
It has been generally recognized that the abnormal transverse diffusion exists not only when an SiO.sub.2 film is used as the selective diffusion mask but also when a phosphosilicate glass (PSG) film is used as a mask or a silicon nitride film is deposited on the substrate. It has not heretofore been possible to effect selective diffusion of impurities with the required degree of accuracy desired in the diffusion pattern.
To summarize, when impurities such as zinc are selectively diffused on the substrate of a compound semiconductor such as GaAs, the above described abnormal transverse diffusion inevitably occurs, so that the accuracy of the diffusion pattern is decreased.